Transient heat assisted sttram cell for lower programming current

ABSTRACT

A memory cell including magnetic materials and heating materials, and methods of programming the memory cell are provided. The memory cell includes a free region, a pinned region, and a heating region configured to generate and transfer heat to the free region when a programming current is directed to the cell. The heat transferred from the heating region increases the temperature of the free region, which decreases the magnetization and the critical switching current density of the free region. In some embodiments, the heating region may also provide a current path to the free region, and the magnetization of the free region may be switched according to the spin polarity of the programming current, programming the memory cell to a high resistance state or a low resistance state.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/642,533, which was filed on Dec. 18, 2009, now U.S. Pat. No.8,238,151, which issued on Aug. 7, 2012.

BACKGROUND

1. Field of Invention

Embodiments of the invention relate generally to magnetic random accessmemory, and more particularly, to Spin Torque Transfer Magnetic RandomAccess Memory (STT-MRAM).

2. Description of Related Art

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the present invention,which are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentinvention. Accordingly, it should be understood that these statementsare to be read in this light and not as admissions of prior art.

Magnetic Random Access Memory (MRAM) is a non-volatile computer memorytechnology based on magnetoresistance. MRAM differs from volatile RandomAccess Memory (RAM) in several respects. Because MRAM is non-volatile,MRAM can maintain memory content when the memory device is not powered.Though non-volatile RAM is typically slower than volatile RAM, MRAM hasread and write response times that are comparable to that of volatileRAM. Unlike typical RAM technologies which store data as electriccharge, MRAM data is stored by magnetoresistive elements. Generally, themagnetoresistive elements in an MRAM cell are made from two magneticregions, each of which holds a magnetization. The magnetization of oneregion (the “pinned region”) is fixed in its magnetic orientation, andthe magnetization of the other region (the “free region”) can be changedby an external magnetic field generated by a programming current. Thus,the magnetic field of the programming current can cause the magneticorientations of the two magnetic regions to be either parallel, giving alower electrical resistance across the magnetoresistive elements (“0”state), or antiparallel, giving a higher electrical resistance acrossthe magnetoresistive elements (“1” state) of the MRAM cell. Theswitching of the magnetic orientation of the free region and theresulting high or low resistance states across the magnetoresistiveelements provide for the write and read operations of the typical MRAMcell.

Though MRAM technology offers non-volatility and faster response times,the MRAM cell is limited in scalability and susceptible to writedisturbances. The programming current employed to switch between highand low resistance states across the MRAM magnetic regions is typicallyhigh. Thus, when multiple cells are arranged in an MRAM array, theprogramming current directed to one memory cell may induce a fieldchange in the free region of an adjacent cell. This potential for writesdisturbances, also known as the “half-select problem,” can be addressedusing a spin torque transfer technique.

A conventional spin torque transfer MRAM (STT-MRAM) cell may include amagnetic cell stack, which may be a magnetic tunnel junction (MTJ) or aspin valve structure. An MTJ is a magnetoresistive data storing elementincluding two magnetic regions (one pinned and one free) and nonmagneticregion in between, which may be accessed through data lines (e.g., bitline, word line, source line, etc.) and an access transistor. A spinvalve has a structure similar to the MTJ, except a spin valve has aconductive region in between the two magnetic regions.

A programming current typically flows through the access transistor andthe magnetic cell stack. The pinned region polarizes the electron spinof the programming current, and torque is created as the spin-polarizedcurrent passes through the stack. The spin-polarized electron currentinteracts with the free region by exerting a torque on the free region.When the torque of the spin-polarized electron current passing throughthe stack is greater than the critical switching current density(J_(c)), the torque exerted by the spin-polarized electron current issufficient to switch the magnetization of the free region. Thus, themagnetization of the free region can be aligned to be either parallel orantiparallel to the pinned region, and the resistance state across thestack is changed.

The STT-MRAM has advantageous characteristics over the MRAM, because thespin-polarized electron current eliminates the need for an externalmagnetic field to switch the free region in the magnetoresistiveelements. Further, scalability is improved as the programming currentdecreases with decreasing cell sizes, and the write disturbance andhalf-select problem is addressed. Additionally, STT-MRAM technologyallows for a higher tunnel magnetic resistance ratio, meaning there is alarger ratio between high and low resistance states, which may improvethe accuracy of read operations in the magnetic domain.

However, high programming current densities through the STT-MRAM cellmay still be problematic. High current densities through the magneticregions of the memory cell may increase the energy consumption in thecell and the thermal profile in the regions, affecting the cell'sintegrity and reliability, and may also lead to larger silicon realestate consumption for each cell.

BRIEF DESCRIPTION OF DRAWINGS

Certain embodiments are described in the following detailed descriptionand in reference to the drawings in which:

FIG. 1 is a graph illustrating the relationship between programmingcurrent direction and the resistance states in an STT-MRAM cell, inaccordance with an embodiment of the present technique;

FIG. 2 is a graph illustrating the relationship between temperature andmagnetization in magnetic materials, in accordance with embodiments ofthe present technique;

FIGS. 3A and 3B depict an axial view and a three-dimensional view of anSTT-MRAM cell configured for heat-assisted programming, in accordancewith embodiments of the present technique; and

FIGS. 4-6 depict axial views of different embodiments of STT-MRAM cellsconfigured for heat-assisted programming, in accordance with embodimentsof the present technique.

DETAILED DESCRIPTION

A spin torque transfer magnetic random access memory (STT-MRAM) cell isprogrammed by changing the resistance of an STT-MRAM structure. TheSTT-MRAM structure, which may also be referred to as an STT-MRAM cellstructure, an STT-MRAM cell, a memory cell, or a magnetic cellstructure, may include regions of materials, including magneticmaterials which may have a magnetization. During programming, onemagnetic region of the STT-MRAM cell, referred to as the “free region,”may be switched in magnetization, and another magnetic region, referredto as the “pinned region,” may remain fixed in magnetization. Typically,the free region magnetization may be in a direction either parallel orantiparallel to the pinned region magnetization. When the magnetizationsof the free and pinned regions are parallel, the resistance across theregions may be low, and when the magnetizations of the free and pinnedregions are antiparallel, the resistance may be high. Thus, an STT-MRAMcell may be programmed to either a low or a high resistance state byswitching the magnetization of the free region.

In a read operation of the STT-MRAM cell, a current is used to detectthe programmed state by measuring the resistance through the cell. Toinitiate a read operation, a read current may be generated and passedthrough a bit line and a source line of the cell and through atransistor. The programmed state of the STT-MRAM structure may bedetermined by the voltage difference between the bit line and the sourceline. In some embodiments, the voltage difference may be compared to areference and amplified by a sense amplifier.

During a write operation of an STT-MRAM cell, a programming current isapplied to the cell that is selected for programming. To initiate thewrite operation, a write current may be generated and passed to the bitline and the source line of the memory cell. As the programming currentpasses the pinned region of the cell, the electrons of the programmingcurrent are spin-polarized by the pinned region to exert a torque on thefree region, which switches the magnetization of the free region to“write to” or “program” the cell. The polarity of the voltage betweenthe bit line and the source line determines the switch in magnetizationof the free region in the cell.

Switching the free region magnetization (and the resistance state of thecell) occurs when the current density passing through the memory cell islarger than the critical switching current density. An example of howthe resistance across a magnetic cell structure in an STT-MRAM cell maychange based on a programming current is depicted in the graph ofFIG. 1. The values used in this graph are examples to illustrate ageneral relationship between a programming current and STT-MRAM cellresistance states. STT-MRAM cells, in embodiments of the presenttechnique, may be programmed with different current values, and may havevarious resistance values in different programmed states. In the graphof FIG. 1, the cell is programmed to a high resistance state R_(HIGH) atapproximately 130 ohms when the programming current is below −1 mA. Thecell is programmed to a low resistance state R_(LOW) at approximately111 ohms when the programming current is above 1 mA. The negative andpositive programming current values may indicate that the programmingcurrent is applied in opposite directions through the magnetic cellstack. Programming currents in opposite directions may have electronswith spin polarization directions that switch the free regionmagnetization in opposite directions (i.e., parallel or antiparallel tothe pinned region magnetization).

When the current through the cell is not below −1 mA or not above 1 mA,then the programming current may not be great enough to switch the freeregion magnetization. More specifically, the current density in the freeregion may not reach the critical switching current density of the freeregion. If the programming current does not have a current density inthe free region that is sufficient to switch the magnetization, the cellmay be at either resistance state, as indicated by hysteresis segmentsH_(HIGH) and H_(LOW) when the current is between −1 mA and 1 mA.

Thus, to program the cell, the programming current density need only beslightly higher than the critical switching current density. Sincepassing a larger programming current increases the energy consumptionand the thermal profile in the cell stack, which affects the integrityand reliability of the cell, it is desirable to decrease the criticalswitching current without affecting the cell's thermal stability.Applying a lower programming current while maintaining a programmingcurrent density that is above the critical switching current densitywould allow a smaller current to switch the free region of the cell. Thefollowing discussion describes the systems and devices, and theoperation of such systems and devices in accordance with the embodimentsof the present technique.

The critical switching current density needed to switch themagnetization of a magnetic material may be directly proportional to themagnetization of the material. This relationship may be presented in thefollowing equation:

$J_{c} = \frac{2{eaM}_{s}{t_{F}\left( {H + H_{K} + {2\pi \; M_{s}}} \right)}}{\hslash\eta}$

where J_(c) represents the critical switching current density, and M_(s)represents the magnetization of a magnetic material. When magnetizationis decreased, the critical switching current density is also decreased.

One method of decreasing the magnetization of the free region todecrease the critical switching current density may be to increase thetemperature of the free region. In some embodiments of the presenttechniques, an STT-MRAM structure may include a heater material, whichmay generate a transient heat which may reduce the magnetization of thefree region. The graph 70 of FIG. 2 plots a relationship (indicated bycurve 76) between temperature (in Kelvins) 72 and magnetization (inelectromagnetic units per gram) 74 in a magnetic material. As depictedby the shape of the curve 76, the magnetization 74 of a magneticmaterial may decrease substantially with an increase of temperature 72towards and past the Curie point (approximately at point 78) of themagnetic material. As the magnetization of a magnetic materialdecreases, the critical switching current density of the material alsodecreases. Thus, by reducing the magnetization of the free region, asmaller programming current may be used to switch the direction ofmagnetization of the free region. Once the free region is magnetizedaccording to the spin polarity of the programming current electrons, theprogrammed state is written to the STT-MRAM structure.

In some embodiments, an STT-MRAM cell may be configured such that heatfrom a heater material may increase the temperature of a free region inthe magnetic cell structure of the STT-MRAM cell, thus decreasing themagnetization (as discussed with respect to FIG. 2) and decreasing thecritical switching current density in the free region (as discussed withrespect to equation (1)). One embodiment for programming a STT-MRAM cellwith a decreased programming current by implementing a heating effect onthe free region is illustrated in FIGS. 3A and 3B, which depict athree-dimensional view (FIG. 3A) and an axial cross section (FIG. 3B) ofan STT-MRAM cell 80.

In one embodiment, the STT-MRAM cell 80 may include a free region 86 anda pinned region 90 with a nonmagnetic region 88 in between. The pinnedregion 90 may have a magnetization with a fixed or preferredorientation, which is represented by the arrow indicating that themagnetization of the pinned region 90 is oriented to the left. The freeregion 86 has a magnetization which may be switched in either adirection parallel or antiparallel to the magnetization of the pinnedregion 90 (e.g., left or right, respectively), thus changing theresistance across the cell structure 80 and programming the cell toeither a high resistance state or a low resistance state. The memorycell structure 80 may also include a heater material 98, which may beemployed to decrease the magnetization of the free region 86. Someembodiments may also include an insulative region 92 which may insulatethe pinned region 90 from a substrate contact 94 coupled to the magneticcell structure 80, as well as an electrical insulator 96 which mayfacilitate in transferring the heat from the heating region 98 to thefree layer 86 and/or controlling the temperature of one or more regionsof the structure 80.

When an STT-MRAM cell is selected for programming, a programming currentmay be applied to the selected cell via a data line. For example, eachSTT-MRAM cell structure in an electronic system may be coupled to a dataline 82 which may deliver current to a selected cell for read and/orwrite operations. The direction of the arrows illustrated in FIGS. 3Aand 3B may generally depict a direction of the programming current toand through one embodiment of an STT-MRAM cell selected for programming.The current may be directed from the data line 82 (FIG. 3A) to thepinned region 90 of the selected cell. The current may then flow fromthe pinned region 90 through the nonmagnetic region 88 and the freeregion 86 to a top electrode 84 of the selected STT-MRAM cell. Theprogramming current may then flow from the top electrode 84 to theheater material 98, which may result in a transient heating of theheater material 98. The top electrode 84 may also be composed of thesame heater material as the heating region 98.

The transient heat produced in the heater material 98 may increase thetemperature of the free region 86, which may decrease the magnetizationof the free region 86 to facilitate programming the selected STT-MRAMcell with a decreased programming current. The decreased magnetizationmay proportionately decrease the critical switching current density ofthe free region 86, or the current density at which the magnetization ofthe free region 86 may be switched. For example, the critical switchingcurrent density for programming a typical STT-MRAM cell may beapproximately 1 MA/cm². In accordance with the present techniques, thecritical switching current density may be reduced to approximately 0.5mA/cm².

The heat generated by heating region 98 and transferred to the freeregion 86 may be transient, and may only affect the free region 86 of acell selected for programming, and only during programming of theselected cell. The heating region 98 may not transfer a significantamount of heat to the free region 86 once the programming operation iscomplete. Further, in this and in other embodiments of the presenttechniques, the magnetization of the pinned region 96 may not besubstantially affected, as the pinned region may have a fixedmagnetization, a higher curie temperature, and/or a higher magnetizationthan the free region 86. Thus, a programming current may be sufficientlyhigh to affect the magnetization of the free region 86, but notsufficiently high to affect the magnetization of the pinned region 96.

The materials discussed below are examples of some materials which maybe used in embodiments in accordance with the present techniques. Insome embodiments, the free region 86 and the pinned region 90 maycomprise ferromagnetic materials, such as Co, Fe, Ni or its alloys,NiFe, CoFe, CoNiFe, or doped alloys CoX, CoFeX, CoNiFeX (X═B, Cu, Re,Ru, Rh, Hf, Pd, Pt, C), or other half-metallic ferromagnetic materialsuch as Fe3O4, CrO2, NiMnSb and PtMnSb, and BiFeO, for instance. Thefree region 86 and the pinned region 90 may have substantially similarmaterials, or may have different combinations of the materials listedabove. The nonmagnetic region 88 may comprise materials such as Cu, Ag,Ta, Au, CuPt, CuMn, other suitable conductive nonmagnetic materials, anycombination of the above materials, or nonconductive materials such asAC_(x)O_(y), MgO, AlN, SiN, CaO_(x), NiO_(x), Hf_(x)O_(y), Ta_(x)O_(y),Zr_(x)O_(y), NiMnO_(x), Mg_(x)F_(y), SiC, SiO₂, SiO_(x)N_(y), or anycombination of the above materials. The insulative region 92 maycomprise materials such as SiN, SiC, or any other suitable dielectric.The insulative region 92 may also comprise chalcogenide material, suchas GeSe or GeS, or any combination of the above materials, and may havea thickness of approximately 10 nm to 30 nm.

The heater material may be any material capable of generating heat andtransferring heat to the free region 86, and may have relatively highresistivity and be able to withstand high temperatures. The heatermaterial may also be relatively inert, and may not react substantiallywith surrounding materials. For example, the heater material may includerefractory metal nitride, TiN, ZrN, HfN, VN, NbN, TaN, TiAlN, TiSiN,TaSiN, TiCN; carbide, such as TiC, ZrC, HfC, VC, NbC, TaC, Cr₃C₂, Mo₂C,WC, SiC, B₄C; boride, such as TiB₂, ZrB₂, HfB₂, VB₂, NbB₂, TaB₂, CrB₂,Mo₂B₅, W₂B₅, doped silicon, such as WSi_(x), MoSi₂, SnO₂:Sb, carbon,niobium, tungsten, molybdenum; metal alloys, such as NiCr, or anycombination thereof. The heater material may have a thickness betweenapproximately 5 nm-20 nm in a direction perpendicular to a current paththrough the heater material. In some embodiments, the heater material 98may be in direct contact with the cell structure 80, and in otherembodiments, the heater material 98 may be separated from the cellstructure 80 by the electrical insulator 96. In one embodiment, mayinclude silicon dioxide, silicon nitride, or any other insulativematerial suitable for electrically insulating and/or transferring heatto the cell structure 80. Further, in some embodiments, a heat insulator96 that is generally parallel between the heater material 98 and thecell structure 80 may have a perpendicular thickness of approximately 2nm to 10 nm.

Furthermore, any of the above examples (e.g., programming currentranges, critical switching current density ranges, materials and/orthicknesses of regions in the cell structure) may vary, based ondifferent configurations of the STT-MRAM cells or of the STT-MRAM cellconfiguration within an electronic system.

The present techniques may involve different configurations of STT-MRAMstructures. For example, FIG. 4 illustrates another embodiment of anSTT-MRAM structure 100 which may be configured to be programmed with areduced programming current by utilizing a transient heating effect. TheSTT-MRAM cell structure 100 of FIG. 4 includes a free region 86 and apinned region 90 with a nonmagnetic region 88 in between. The pinnedregion 90 may be coupled to a substrate contact 94 of the STT-MRAMarray. In different embodiments, the pinned region 90 may either be indirect contact with the substrate contact 94, or may be insulated fromthe substrate contact 94 by insulative materials. The STT-MRAM structure100 may include heater material 98 which may transfer a programmingcurrent from a data line 82 to the free region 86. The heater material98 may increase the temperature of the free region 86 to decrease themagnetization of the free region 86 and facilitate in switching themagnetization of the free region 86 with the programming current. Insome embodiments, the heater material 98 may be coupled to a heatinsulator 96, which may limit temperature changes in the regions of thecell 100 due to the heat generated in the heater material 98.

As depicted in FIG. 4, the heater material 98 may be disposed over oneor more STT-MRAM structures, and may also be connected to the data lines82 which deliver current to each cell. When a memory cell is selected tobe programmed, the programming current may travel through the dataline(s) 82 corresponding to the selected cell, and through the heatermaterial 98 in contact with the data line(s) 82 of the selected cell.The flow of the programming current through the heater material 98 maycause the heater material 98 to generate a transient heat, which mayincrease the temperature of the free region 86. The transienttemperature increase in the free region 86 may decrease themagnetization, and thus the critical switching current density of thefree region 86. The magnetization of the free region 86 may be switchedaccording to the polarity of the programming current, and theprogramming current may then pass through the nonmagnetic region 88 andthe pinned region 90 to the substrate contact 94. Thus, the sameprogramming current that is directed to the heater material 98 toincrease the temperature of the free region 86 also switches themagnetization based on the spin polarity of the current. Further,because of the decrease in the critical switching current density, alower programming current may be sufficient for switching themagnetization of the free region 86 to program the cell.

In some embodiments, the STT-MRAM cell structure 100 may also include aheat insulator 96, such as silicon dioxide, to protect the thermalprofile of the STT-MRAM structure by insulating the magnetic materialsof the structure 100. For example, the heat insulator 96 may be disposedbetween the heater material 98 and the regions of the structure 100. Insome embodiments, the heat insulator 96 may also be disposed between thefree region 86 and the heater material 98 to control and/or slow theheating of the free region 86.

Another embodiment of the present techniques is presented in FIG. 5,which illustrates an STT-MRAM cell structure 102 having heater material98 disposed perpendicularly to the memory cell regions. The structure102 may include a free region 86 and a pinned region 90 with anonmagnetic region 88 in between the free and pinned regions 86 and 90.The cell may be accessed through a data line 82, which may deliver aprogramming current through the regions (e.g., the free, nonmagnetic,and pinned regions 86, 88, and 90) of the structure 102 to a substratecontact 94. Each structure 102 may also include heater material 98,which also provides a current path between the data line 82 and thesubstrate contact 94 of the cell structure 102.

In some embodiments, the structure 102 may also include heat insulators96, which may insulate portions of the cell structure 102 from theheating material 98. For example, the heat insulators 96 may beconfigured perpendicularly between the regions of the cell structure 102and the heating material 98 to control the temperature of the celland/or facilitate in maintaining cell integrity. In one embodiment, theheating material 98 may be in direct contact with the data line 82 andthe free region 86, and in some embodiments, the cell structure mayinclude buffer nonmagnetic materials, such as Cu, Au, Ta, TaN, TiN Ru,Ag, CuPt, CuMn, other nonmagnetic transition metals, such as Os, Ru, Re,alloys comprising at least two elements of Os, Ru and Re, or anycombination of the above nonmagnetic conductive materials. The buffernonmagnetic materials may be disposed between the free region 86 and theheating material 98, and may eliminate direct contact between the twomaterials to control the temperature changes in the cell structureand/or protect the integrity of the cell.

In another embodiment, an STT-MRAM cell structure 104, as illustrated inFIG. 6, may include heating material 98 disposed between a data line 82and the regions of the cell structure 104. The structure may includeheating material 98, a free region 86, a nonmagnetic region 88, and apinned region 90 over a substrate contact 94 of the cell. Theprogramming current in the heating material 98 may cause the heatingmaterial 98 to generate heat, which increases the temperature of thefree region 86. The increase in temperature in the free region 86 mayresult in a decrease in magnetization, and a proportionate decrease inthe critical switching current density, or the current density in thefree region 86 at which the magnetic materials of the region may beswitched in magnetization. Thus, the heating material 98 may facilitatein delivering the programming current from the data line 82 to the freeregion 86, as well as in reducing the critical switching currentdensity, such that a lower programming current may program the selectedcell.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

1. A method of operating a memory cell comprising: heating a free regionof a magnetic cell structure to increase the temperature of the freeregion; and programming the memory cell while the temperature of thefree region is increased.
 2. The method, as set forth in claim 1,wherein heating the free region comprises transferring heat from aheater material to the free region.
 3. The method, as set forth in claim2, wherein transferring heat from the heater material to the free regionis facilitated by an electrical insulator.
 4. The method, as set forthin claim 1, wherein heating the free region comprises decreasing acritical switching current density of the free region.
 5. The method, asset forth in claim 4, wherein decreasing the critical switching currentdensity of the free region comprises decreasing the critical switchingcurrent density of the free region to approximately 0.5 mA/cm².
 6. Themethod, as set forth in claim 1, wherein heating the free regioncomprises directing a current to the free region.
 7. The method, as setforth in claim 6, wherein the current is directed in a current path,wherein the current path flows from a pinned region in the magnetic cellstructure to the free region to the heater material.
 8. The method, asset forth in claim 6, wherein the current is directed in a current path,wherein the current path flows from the heater material to the freeregion.
 9. The method, as set forth in claim 8, wherein a portion of thecurrent path through the heater material is in a direction perpendicularto the free region and a pinned region in the magnetic cell structure.10. The method, as set forth in claim 6, wherein the current is directedin one or more parallel current paths, wherein a first current path isthrough the heater material, and wherein a second current path isthrough the free region and a pinned region of the magnetic cellstructure.
 11. A magnetic cell structure comprising: a free region; apinned region; and a heater material configured to generate heat andtransfer the heat to the free region when a programming current isdirected to the magnetic cell structure.
 12. The magnetic cellstructure, as set forth in claim 11, comprising an electrical insulatorto facilitate heat transfer from the heater material to the free region.13. The magnetic cell structure, as set forth in claim 11, wherein thefree region and the pinned region comprise ferromagnetic materials. 14.The magnetic cell structure, as set forth in claim 11, comprising anonmagnetic region disposed between the free region and the pinnedregion.
 15. The magnetic cell structure, as set forth in claim 11,comprising an insulative region configured to insulate one or moreregions of the magnetic cell structure from one or more of theprogramming current and the heat generated in the heater material. 16.The magnetic cell structure, as set forth in claim 11, comprising anelectrode coupled to the free region and the heater material, whereinthe electrode is configured to carry the programming current to one ormore regions of the magnetic cell structure.
 17. The magnetic cellstructure, as set forth in claim 11, wherein the heater materialcomprises TiN, ZrN, HfN, VN, NbN, TaN, TiAlN, TiSiN, TaSiN, TiCN, TiC,ZrC, HfC, VC, NbC, TaC, Cr3C2, Mo2C, WC, SiC, B4C, TiB2, ZrB2, HfB2,VB2, NbB2, TaB2, CrB2, Mo2B5, W2B5, WSix, MoSi2, SnO₂:Sb, carbon,niobium, tungsten, molybdenum, metal alloys, or any combination thereof.18. The magnetic cell structure, as set forth in claim 17, arranged suchthat during operation, the programming current is directed to themagnetic cell structure in parallel, wherein one path of the programmingcurrent is directed to the free region and the pinned region, andwherein another path of the programming current is directed to theperpendicularly arranged heater material.
 19. The magnetic cellstructure, as set forth in claim 17, configured to provide a currentpath from the pinned region to the free region to a conductive region tothe heater material.
 20. The magnetic cell structure, as set forth inclaim 11, configured to provide a current path from the heater materialto the free region to the pinned region.
 21. The magnetic cellstructure, as set forth in claim 11, wherein a spin-torque transfermagnetic random access memory cell comprises the magnetic cellstructure.
 22. A method of operating a memory cell comprising:decreasing a critical switching current density of a free region of amagnetic cell structure; and programming the memory cell afterdecreasing the critical switching current density of a free region of amagnetic cell structure.
 23. The method, as set forth in claim 22,wherein decreasing the critical switching current comprises heating thefree region.
 24. The method, as set forth in claim 23, wherein heatingthe free region comprises transferring heat from a heater material tothe free region.
 25. The method, as set forth in claim 24, whereintransferring heat from the heater material to the free region isfacilitated by an electrical insulator.
 26. The method, as set forth inclaim 22, wherein decreasing the critical switching current density ofthe free region comprises decreasing the critical switching currentdensity of the free region to approximately 0.5 mA/cm².